Excess Solute Carbon and Tetragonality in As-Quenched Fe-1Mn-C (C:0.07 to 0.8 Mass Pct) Martensite

Abstract

The carbon distribution behavior and crystal structure of as-quenched martensite in Fe-1Mn-C (C: 0.07 to 0.8 mass pct) steels were quantitatively investigated by atom probe tomography (APT) and X-ray diffraction with Rietveld analysis. APT revealed that the martensite steels contained quantities of carbon in solid solution far beyond its solubility in body-centered cubic (bcc)-Fe in all the alloys investigated; the carbon atoms were non-homogeneously distributed as carbides or aggregates on dislocations due to autotempering. Tetragonality was observed in the steels with interstitial solute carbon concentrations in the range of 0.1 to 0.7 mass pct, but was not evident below 0.1 mass pct. The appearance and disappearance of tetragonality in the low-carbon steels may be explained by the disordered bcc ↔ ordered body-centered tetragonal (bct) mechanism, considering the partial tetragonality due to the heterogeneity of the interstitial solute carbon distribution. The existence of tetragonality in the autotempered low-carbon steels can alternatively be understood by a mechanism based on the kinetic decrease of tetragonality during cooling, where the microscopic strain release is the rate-controlling process. The excess carbon solubility in the autotempered low- and medium-carbon martensite is due to the existence of tetragonal distortions, owing to the slow kinetics of the tetragonality decrease during cooling.